Dual chamber intracardiac medical device

ABSTRACT

An implantable pacemaker has a first housing and a second housing tethered to the first housing by an elongated electrical conductor. The elongated electrical conductor has a proximal end coupled to the first housing and a distal end coupled to the second housing and includes a signal line configured to carry an electrical signal between the first housing and the second housing.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to co-pending and commonly-assignedU.S. patent application Ser. No. ______ (Atty. Docket No.:C00001479.USP2) which is entitled DUAL CHAMBER INTRACARDIAC MEDICALDEVICE, which is filed concurrently herewith and is incorporated hereinby reference in its entirety.

TECHNICAL FIELD

The disclosure relates to an implantable wholly intracardiac medicaldevice for delivering cardiac pacing and/or sensing cardiac signals.

BACKGROUND

Implantable cardiac pacemakers are often placed in a subcutaneous pocketand coupled to one or more transvenous medical electrical leads carryingpacing and sensing electrodes positioned in the heart. A cardiacpacemaker implanted subcutaneously may be a single chamber pacemakercoupled to one medical lead for positioning electrodes in one heartchamber, atrial or ventricular, or a dual chamber pacemaker coupled totwo leads for positioning electrodes in both an atrial and a ventricularchamber. Multi-chamber pacemakers are also available that may be coupledto three leads, for example, for positioning electrodes for pacing andsensing in one atrial chamber and both the right and left ventricles.

Intracardiac pacemakers have recently been introduced that are whollyimplantable within a ventricular chamber of a patient's heart fordelivering ventricular pacing pulses. Such a pacemaker may sense R-wavesignals attendant to intrinsic ventricular depolarizations and deliverventricular pacing pulses in the absence of sensed R-waves. While singlechamber ventricular pacing may adequately address some patientconditions, other conditions may require atrial and ventricular pacing,commonly referred to as a dual chamber pacing, in order to maintain aregular heart rhythm.

SUMMARY

In general, the disclosure is directed to an intracardiac dual chamberpacemaker. The pacemaker includes a first housing and a second housingtethered to the first housing by an electrical conductor. The first andsecond housings each include a pulse generator for delivering pacingpulses to spaced apart pacing sites of a patient's heart.

In one example, the disclosure provides an implantable pacemakercomprising a first housing enclosing a first pulse generator configuredfor delivering first pacing pulses to a first heart location via atleast one first electrode carried by the first housing and a secondhousing enclosing a second pulse generator configured for deliveringsecond pacing pulses to a second heart location spaced apart from thefirst heart location via at least one second electrode carried by thesecond housing. The pacemaker further includes an elongated electricalconductor having a proximal end and a distal end, the proximal endcoupled to the first housing and the distal end coupled to the secondhousing to tether the first housing and the second housing together. Theelongated electrical conductor includes a signal line configured tocarry an electrical signal between the first housing and the secondhousing.

In another example, the disclosure provides a method performed by animplantable pacemaker comprising a first housing enclosing a first pulsegenerator, a second housing enclosing a second pulse generator. Thefirst and second housings are tethered together by an elongatedelectrical conductor having a proximal end coupled to the first housingand a distal end coupled to the second housing. The method includesdelivering first pacing pulses to a first heart location via at leastone first electrode carried by the first housing, delivering secondpacing pulses to a second heart location spaced apart from the firstheart location via at least one second electrode carried by the secondhousing, and transferring a signal along a signal line extending throughthe elongated electrical conductor between the first housing and thesecond housing to enable coordinated delivery of the first pacing pulsesand the second pacing pulses.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control module of an implantable pacemaker, cause thepacemaker to deliver first pacing pulses to a first heart location by afirst pulse generator enclosed in a first housing via at least one firstelectrode carried by the first housing, deliver second pacing pulses toa second heart location by a second pulse generator enclosed in a secondhousing via at least one second electrode carried by the second housing;and transfer a signal via an elongated electrical conductor extendingfrom the first housing to the second housing to enable coordinateddelivery of the first pacing pulses and the second pacing pulses.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an intracardiac dual chamberpacemaker that may be used to sense cardiac electrical signals andprovide therapy to a patient's heart.

FIG. 2A is a conceptual diagram of the intracardiac pacemaker shown inFIG. 1.

FIG. 2B is a conceptual diagram of an intracardiac pacemaker includingone capsule having a greater fixation force.

FIG. 3 is a conceptual diagram of the pacemaker shown in FIG. 1including pace/sense electrodes along an electrical conductor thattethers the separate housings of the pacemaker.

FIG. 4 is a conceptual diagram of an alternative arrangement of thepacemaker of FIG. 1.

FIGS. 5A and 5B are exemplary sectional views of the tetheringelectrical conductor of the pacemaker of FIG. 1.

FIG. 6 is a functional block diagram of the pacemaker of FIG. 1according to one example.

FIG. 7 is a functional block diagram of the pacemaker of FIG. 1according to another example configuration.

FIG. 8 is a functional block diagram of yet another exampleconfiguration of the pacemaker of FIG. 1.

FIG. 9 is a flow chart of a method performed by the pacemaker of FIG. 1according to one example.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram illustrating an intracardiac dual chamberpacemaker 10 that may be used to sense cardiac electrical signals andprovide therapy to a patient's heart 8. Pacemaker 10 is an elongateddevice that includes a right atrial (RA) intracardiac capsule 12 and aright ventricular (RV) intracardiac capsule 14. The capsules 12 and 14are tethered together by electrical conductor 16. Pacemaker 10 is shownextending within the right atrium and the right ventricle of heart 8,but could be deployed to extend within the left atrium and the leftventricle.

In the example shown, RA capsule 12 is configured to sense RA cardiacsignals and deliver RA pacing pulses via a pair of RA electrodes, whichmay be housing based electrodes incorporated along the housing ofcapsule 12. RV capsule 14 is configured to sense RV cardiac signals anddeliver RV pacing pulses via a pair of RV electrodes, which may behousing based electrodes incorporated along the housing of capsule 14.

RA capsule 12 may include a fixation member 30 to retain capsule 12 in adesired location within the right atrium, and RV capsule 14 may includea fixation member 32 to retain capsule 14 in a desired location withinthe right ventricle. In the example of FIG. 1, capsule 12 is positionedalong an endocardial wall of the RA, e.g., along the RA lateral wall,but could be positioned along the RA septum or other locations. Capsule14 is positioned along an endocardial wall of the RV, e.g., near the RVapex. Pacemaker 10, however, is not limited to being positioned in thelocations shown in the example of FIG. 1 and other positions andrelative locations of capsules 12 and 14 with respect to one another andheart 8 are possible and will be selected based at least in part onparticular patient need.

Capsules 12 and 14 are reduced in size compared to typical,subcutaneous-type pacemakers and may be generally cylindrical in shapeto enable transvenous implantation of pacemaker 10 via a deliverycatheter. The electrical conductor 16 may extend between any twolocations of capsules 12 and 14 and is provided with a length that isadequate to reach between the two desired implant sites of capsules 12and 14 without undue tension that would increase the likelihood ofdislodgment or dislocation of either of the capsules 12 and 14. Invarious examples, without limitation, electrical conductor 16 may have alength between 5 cm and 25 cm in length.

In other examples, pacemaker 10 may be positioned along the outside ofheart 8, including epicardial or pericardial locations. Capsule 12 maybe positioned at one pacing site and capsule 14 positioned at a secondpacing site spaced apart from the first pacing site, along the same ortwo different cardiac chambers to provide multi-site or dual chamberpacing therapy. For example, capsule 12 may be positioned outside alongthe right atrium or left atrium to provide respective right atrial orleft atrial sensing and pacing. Capsule 14 may be positioned outsidealong the right ventricle or left ventricle to provide respective rightventricular or left ventricular pacing and sensing.

Capsules 12 and 14 each include pacing circuitry, e.g., a pacing pulsegenerator, for delivering electrical stimulation pulses, i.e., pacingpulses, to heart 8 via associated electrodes on the outer housings ofcapsules 12 and 14. The RA pacemaker 12 and the RV pacemaker 14 areconfigured to control the delivery of pacing pulses to the respectiveatrial and ventricular chambers in a manner that promotes coordinateddual chamber pacing. Examples of dual chamber pacing therapy deliveredby separate atrial and ventricular intracardiac pacemakers are generallydisclosed in commonly-assigned U.S. Pat. App. Publication No.2014/0121720 (Bonner, et al.), incorporated herein by reference in itsentirety. By including the tethering electrical conductor 16 betweencapsules 12 and 14, a signal may be transmitted between the capsules 12and 14 to enable coordinated dual chamber pacing therapy at the twospaced apart pacing sites. The signal carried by tethering electricalconductor 16 may be a power transmission signal or a data communicationsignal in some examples. Frequent wireless telemetry communicationsignals are not required, which reduces the overall power capacityrequired by each of the separate capsules 12 and 14.

Pacemaker 10 is capable of bidirectional wireless communication with anexternal device 20. External device 20 may be a programmer used by aclinician or other user in a medical facility, a home monitor located ina patient's home, or a handheld device. Aspects of external device 20may generally correspond to the external programming/monitoring unitdisclosed in U.S. Pat. No. 5,507,782 (Kieval, et al.), herebyincorporated herein by reference in its entirety.

External device 20 may be configured to establish separate wirelessradio frequency (RF) communication links 22 and 24 with separateimplantable telemetry modules included in each of RA capsule 12 and RVcapsule 14 to separately program and interrogate the individual RAcapsule 12 and RV capsule 14 for respective RA pacing and sensingfunctions and RV sensing and pacing functions. In other examples, animplantable telemetry module is included in only one of RA capsule 12and RV capsule 14 and a data line extends between the capsules 12 and 14within electrical conductor 16 for transmitting data to and fromexternal device 20 via a single implantable telemetry module.

An example RF telemetry communication system that may be implemented inexternal device 20 and pacemaker 10 is generally disclosed in U.S. Pat.No. 5,683,432 (Goedeke, et al.), hereby incorporated herein by referencein its entirety. Communication links 22 and 24 may be established usinga radio frequency (RF) link, for example in the Medical ImplantCommunication Service (MICS) band, Medical Data Service (MEDS) band,BLUETOOTH® or Wi-Fi.

External device 20 may be capable of bi-directional communication withpacemaker 10 over a wide range of distances, e.g., up to approximately10 meters. In other examples, telemetry communication may require theuse of a programming head placed in proximity of the patient, e.g.against or within several centimeters of the patient's skin or clothing,to facilitate data transfer.

It is contemplated that external device 20 may be in wired or wirelessconnection to a communication network for transferring data to a remotedatabase or computer to allow remote management of the patient. Anexample communication scheme that may be used for remotely programmingpacemaker 10 using the techniques disclosed herein is generallydisclosed in U.S. Pat. No. 6,442,433 (Linberg), incorporated herein byreference in their entirety.

External device 20 may be used for retrieving and sending data frompacemaker 10. Examples of retrieved data include physiological signalssuch as RA or RV EGM signals, therapy delivery data such as a history ofpacing frequency, results of device diagnostic testing, currentoperating control parameters or other data stored by the pacemaker. Datasent to pacemaker 10 may include programmable control parameters used bythe RA capsule 12 and RV capsule 14 to control sensing and pacingfunctions.

Both capsules 12 and 14 may include separate implantable telemetrymodules configured to periodically “listen” for a valid “wake up”telemetry signal from external device 20 and power up to establish acommunication link 22 or 24 in response to a valid RF telemetry signal(or go back to “sleep” if no valid telemetry signal is received).However, capsules 12 and 14 may not be configured to transmit a “wakeup” signal to the other capsule 12 or 14 to initiate a communicationsession. In some cases, electrical conductor 16 includes a data linethat enables data signals to be transmitted between RA capsule 12 and RVcapsule 14. In other examples, electrical conductor 16 is solely a powertransmission cable and does not include a data transmission line. Inthese instances, the capsules 12 and 14 may or may not be configured tocommunicate wirelessly with each other via the separate telemetrymodules. For example, neither RA capsule 12 nor RV capsule 14 may beconfigured to initiate an RF communication session with the othercapsule. In other cases, the two capsules 12 and 14 may be configured tocommunicate wirelessly with each other, but, in order to conservebattery life of the pacemaker 10, wireless telemetry communicationbetween capsules 12 and 14 may be minimized.

FIG. 2 is a conceptual diagram of intracardiac pacemaker 10 shown inFIG. 1. Pacemaker 10 includes capsule housing 50 and capsule housing 60tethered together by electrical conductor 16. Housing 50 is shown havinga smaller outer dimension than housing 60. In some examples, housing 50has the same or similar diameter but a shorter length than housing 60 asshown in FIG. 2. The relative lengths, outer diameters, and sizes ofcapsules 12 and 14 are not necessarily drawn to scale in FIG. 2 and mayvary between embodiments. In other examples, housing 50 may have thesame or similar length as housing 60 but a smaller outer diameter. Instill other examples, housing 50 and housing 60 may have approximatelythe same outer dimensions or may differ in both length and outerdiameter. Depending on the intended implant locations of capsules 12 and14, the overall dimensions of housing 50 and 60 may be adaptedaccordingly.

Capsules 12 and 14 are generally cylindrical in shape to facilitatedelivery of pacemaker 10 via a catheter or other generally tubulardelivery tool. For example, capsule 14 may be positioned near a distalopening of a delivery catheter with electrical conductor 16 and capsule12 extending within the catheter lumen in a proximal direction from thecatheter distal opening. In this way, pacemaker 10 may be delivered byfirst releasing capsule 14 from the catheter distal opening at a firstdesired implant location, e.g., in or along the right ventricle. Thedelivery catheter may then be withdrawn to release electrical conductor16 and position the distal opening of the delivery catheter at a seconddesired implant location. Capsule 12 may then be released at the secondimplant location, e.g., in the right atrium. It is recognized however,that capsules 12 and 14 may be provided with other outer geometries,such as generally prismatic shapes, spherical, etc.

In the illustrative embodiment of FIG. 1, capsule 12 is intended for RAimplantation and is provided with a smaller length than capsule 14intended for RV implantation. Housing 50 may include a controlelectronics subassembly 52, and housing 60 may include a controlelectronics subassembly 62 assembled with a battery subassembly 64. Thebattery subassembly 64 of capsule 14 may provide power to the both thecontrol electronics subassembly 62 of capsule 14 and the controlelectronics assembly 52 of capsule 12 enabling the overall volume ofhousing 50 to be smaller in than the overall volume of housing 60. Inthis example, electrical conductor 60 includes an insulated power supplyline and a ground wire extending from the battery subassembly 64 to thecontrol subassembly 52 of capsule 12.

Capsule 12 has a proximal housing end 56 and a distal housing end 58.Proximal housing end 56 is shown as a substantially flat portion ofhousing 50 but may be a more rounded or substantially hemisphericalproximal end. A proximal electrode 42 may be provided as a ringelectrode along housing 50, near proximal end 56. Proximal electrode 42may be an uninsulated portion of housing 50 or electrically coupled tohousing 50 to serve as an anode return electrode of a bipolar pacing andsensing electrode pair including electrodes 40 and 42. Distal housingend 58 is defined by control electronics subassembly 52 and a tipelectrode 40. Tip electrode 40 is provided as the cathode electrode fordelivering pacing pulses and is coupled to a pulse generator enclosed incontrol electronics subassembly 52 in some examples. Tip electrode 40may also be coupled to a sensing module enclosed in control electronicssubassembly 52 for sensing cardiac electrical signals, i.e., EGMsignals.

Capsule 14 has a proximal housing end 66 and a distal housing end 68.Proximal housing end 66 is shown as a substantially flat portion ofhousing 60 but may be a rounded or substantially hemispherical proximalend. A proximal electrode 46 may be provided as a ring electrode alonghousing 60, near proximal end 66. Proximal electrode 46 may be anuninsulated portion of housing 60 or electrically coupled to housing 60to serve as an anode return electrode of a bipolar pacing and sensingelectrode pair including electrodes 44 and 46. Distal housing end 68 isdefined by control electronics subassembly 62 and a tip electrode 44.Tip electrode 44 is provided as the cathode electrode for deliveringpacing pulses and sensing EGM signals and is coupled to a pulsegenerator and sensing module enclosed in control electronics subassembly62.

An active or passive fixation member 30 and 32 may be provided near eachof distal housing ends 58 and 68, respectively, to maintain a stableposition of respective tip electrodes 40 and 44 at desired pacing andsensing sites, e.g., within the right atrium and within the rightventricle respectively. As used herein, the terms “active” and “passive”with reference to a fixation member are used to describe the mechanicalfixation function of the fixation member. The term “active fixation”refers to fixation of the respective fixation member within tissue atthe implant site by intentionally piercing, perforating or penetratingthrough a tissue surface by the fixation member at the time ofimplantation.

In contrast, “passive fixation” refers to fixation of the respectivemember at an in implant site that does not include intentionallypiercing, perforating or penetrating through a tissue surface at thetime of implantation. Rather, “passive fixation” involves positioningthe fixation member alongside or adjacent to tissue at the implant siteand relying on passive interaction or engagement between the passivefixation member and the adjacent tissue structure(s) to passively holdthe fixation member in place. An active fixation member, therefore,includes a sharp or piercing tip that enables penetration into a tissuesurface whereas a passive fixation member does not include a sharp orpiercing tip and may include rounded or blunt tips that are not intendedto be inserted into a tissue surface.

Each of fixation members 30 and 32 may include multiple fixation tinesas shown in FIG. 2. For example each of fixation members 30 and 32 mayinclude two, three, four, six or more fixation tines. The individualtines may extend in a generally distal direction with respect to housingdistal ends 58 and 68 from a fixed tine end 34 or 36 coupled to distalhousing end 58 or 68, then curve or bend laterally and proximally toextend a free, terminal tine end 35 or 36 in a substantially proximaldirection with respect to distal housing end 58 or 68. The fixationmembers 30 and 32 may be active fixation members configured to pierceinto endocardial tissue to hold the respective capsule 12 or 14 in astable position. In this case, tine free ends 35 and 38 are pointed topierce through the tissue at the implant site. In other examples, one orboth of fixation members 30 and 32 may be a passive fixation member thatpassively interacts with tissue at the implant site, e.g., the atrialpectinate muscles or the ventricular trabeculae, respectively, tomaintain a stable position of tip electrode 40 or 42. In this case, tinefree ends 35 and 38 may be rounded or blunt.

In one example, capsule 14 is configured to have a greater fixationforce than capsule 12. The fixation force is the force required todislodge or retract the capsule 14 or 12 from its respective implantsite, e.g. by pulling on respective proximal housing ends 66 or 56 or onelectrical conductor 16. In this way, if heart growth, repetitive motionor other forces produce tension along electrical conductor 16 betweencapsules 12 and 14, capsule 12 will preferentially dislodge from itsimplant location before (or instead of) capsule 14, allowing capsule 14to remain stably located at its implant site.

The capsule 12 or 14 considered to be more critical in deliveringtherapy to heart 8 may be provided with a greater fixation force. Insome examples, the more critical capsule may be capsule 14 positioned inthe right ventricle as shown in FIG. 1 for delivering RV pacing in apatient with AV block. In other examples, the more critical capsule maybe capsule 12 positioned in the right atrium for delivering bradycardiapacing in a patient with intact AV conduction, where capsule 14 mayprovide occasional back-up ventricular pacing only or ventricular pacingduring atrial tachyarrhythmia, for example.

A higher fixation force for one of capsules 12 or 14 compared to theother of capsules 12 and 14 may be achieved by configuring one of thefixation members 30 or 32 with a greater fixation force. The fixationmember 30 or 32 having a greater fixation force may have, for examplebut not limited to, a greater length, thicker diameter, greater materialstiffness, or greater number of tines than the other of fixation members30 or 32. Additionally or alternatively, fixation members 30 and 32 mayhave different shapes such as a sharper curvature or bend of fixationmember tines that results in a greater pulling force required todislodge one of the fixation members 30 or 32 from tissue at its implantsite compared to the force required to dislodge the other one offixation members 30 and 32 from its implant site.

In other examples, one of fixation members 30 or 32 is provided as anactive fixation member and the other of fixation members 30 or 32 isprovided as a passive fixation member that is more easily dislodged froman implant location than the active fixation member. An active fixationmember may include a piercing tip, helical screw, or one or more activefixation tines as generally disclosed in U.S. Patent Publication No.2014/0180306 (Grubac, et al.), incorporated herein by reference in itsentirety. For example distal fixation member 32 may be an activefixation member including multiple fixation tines which may beperforating nitinol wires and proximal fixation member 30 may be passivepolymer tines that do not pierce into tissue. Fixation member 30 mayinclude tines that extend at a straight angle from capsule 12 instead ofcurving as shown in FIG. 2.

The active fixation tines of distal fixation member 32 may be held in asubstantially straight, distally-extended position within a deliverytool or catheter such that the free tine ends 38 pierce into tissue atthe implant site upon deployment from the delivery tool. The fixationtines may resume the unextended, pre-formed curved position shown inFIG. 2 upon full deployment from the delivery tool, actively engagingtissue at the implant site by perforating through the tissue andcapturing tissue within at least the curved portion of the tines as theyresume the unextended position shown.

Fixation members 30 and 32 may be formed from a biocompatible polymer,e.g., polyurethane, silicone, polyethylene, or polyether ether ketone,or from a metal or metal alloy, e.g., stainless steel, titanium,platinum, iridium, tantalum, nickel or alloys thereof, or a coatedmetal. Fixation members 30 and/or 32 may include a shape memory materialsuch as Nitinol to retain a pre-formed bend or curve that isstraightened when pacemaker 10 is placed in a delivery catheter or tooland restored after pacemaker 10 is released from the delivery catheteror tool.

Capsules 12 and 14 are shown tethered together by electrical conductor16 at their respective proximal housing ends 56 and 66. In other words,electrical conductor 16 extends from its proximal end 17 coupled toproximal housing end 56 of capsule 12 to its distal end 18 coupled toproximal housing end 66 of capsule 14. Electrical conductor 16 may befixedly coupled to proximal housing ends 56 and 66 in a permanent,non-removable manner. In other examples, electrical conductor 16 may beremovable by a user and re-attachable at one or both of proximal end 17and distal end 18.

Electrical conductor 16 includes an electrically-insulating elongatedbody 15 that encloses one or more lumens through which electricallyconductive wires, cables or traces extend between capsules 12 and 14 forcarrying power and/or data communication signals between capsules 12 and14. In some examples, electrical conductor 16 is provided forexclusively conducting power and/or communication signals betweencapsules 12 and 14. In other examples, electrical conductor 16 mayinclude one or more electrodes exposed along elongated body 15 for usein sensing cardiac electrical signals and/or delivering cardiac pacingpulses to heart 8 generated by either of capsules 12 or 14.

FIG. 2B is a conceptual diagram of pacemaker 10 including one capsule 14having a greater fixation force created by concave surface of housing60′. In various examples, one of capsules 12 or 14 desired to have agreater fixation force may have a housing 50 or 60 that includes one ormore concave surfaces that create a greater fixation force over time astissue encapsulation of the capsule 12 or 14 takes place. Tissueencapsulation along or within a concave surface or annular feature ofhousing 50 or 60 creates a stronger resistance against retraction forcesthan tissue encapsulation around a laterally flat or convex surface.

In the example shown, housing 60′ includes a circumferential concavity61 along a portion of battery subassembly 64. In other examples, aconcavity may be created along any portion of a longitudinal surface,e.g., the longitudinal sidewall extending between the proximal anddistal ends 66 and 68 of housing 60,′ to promote a narrowing orsqueezing in of the tissue encapsulation along the capsule 14 that issubstantially transverse to the direction of a longitudinal retractionforce, e.g., along electrical conductor 16. Concavity 61 is shown nearthe housing distal end 68 which is expected to encapsulate earlier thanhousing proximal end 66 after implantation, thereby promoting earlyencapsulation of the concavity 61 and resistance to retraction forces.The growth of encapsulating tissue generally toward a central axis ofhousing 60′, transverse to a longitudinal retraction force, will resistthe retraction force. Concavity 61′ is not necessarily drawn to scalewith respect to housing 60′ and may have a different radius defining thecurvature of concavity 61 and/or a different depth from the exteriorsurface of longitudinal sidewall, relative to the overall diameter andlength of housing 60′, than as depicted in FIG. 2B.

Housing proximal end 66 is shown a substantially lateral flat surfacethat is transverse to the direction of a longitudinal retraction forceand will further resist retraction and dislodgment of the housing 60′.Housing 50′ includes a rounded convex or cone-shaped distal end 56′extending from substantially flat longitudinal sides. All exteriorsurfaces of housing 50′ are substantially convex surfaces orlongitudinally flat surfaces making the retraction force required todislodge capsule 12 from tissue encapsulation at an implant site lessthan the retraction force required to dislodge capsule 14. The laterallyflat distal end 66 and the concavity 61 of capsule 14 will increase theretraction force required to remove capsule 14 from an implant sitecompared to the retraction force required to remove capsule 12.

In some cases, all or a portion of housing 60′ is covered by a coatingthat promotes tissue encapsulation or ingrowth for producing a greaterfixation force of capsule 14. For example a generally non-smooth coatingthat creates pits, pores, ridges or bumps may promote a thicker tissueencapsulation and/or tissue ingrowth that result in a great fixationforce. In one example, at least a portion of housing 60′ is covered byDACRON® mesh or another synthetic porous material. A coating on all or aportion of housing 60′ that is hydrophobic may promote macrophageadhesion and fusion leading to more rapid development of tissueencapsulation of the capsule 14.

Additionally or alternatively, capsule 12 may include a coating over allor a portion of housing 50′ that has a surface chemistry and morphologythat reduces encapsulation. For example, a coating on capsule 12 may besmoother, e.g., less porous, be a relatively more hydrophilic or neutralsurface than the surface of housing 60′, and/or includeanti-inflammatory agents such that tissue encapsulation of capsule 12 isslower than or ultimately has a final encapsulation thickness that isless than tissue encapsulation of capsule 14. In various examples, withno limitation intended, a coating on housing 50′ may include ananti-inflammatory drug such as dexamethasone, a synthetic coating suchas polytetrafluroethylene (PTFE), poly-lactic acid, polylacticcoglycolic acid (PLGA), 2-hydroxyethyl methacrylate, polyethyleneglycol, polyvinyl alcohol, or a phospholipid-containing material thatreduces adhesion of inflammatory cells to housing 30′.

In FIG. 2B, capsule 12 is shown having a fixation member 30′ withshorter fixation tines that will reduce the fixation force of capsule 12relative to the longer fixation member tines of fixation member 32.Accordingly, greater fixation force of one capsule 14 relative to theother capsule 12 may be provided by including a concavity 61 of housing60′, an outer surface geometry and/or surface chemistry that promotesfaster tissue encapsulation than the outer surface of housing 50′,and/or a fixation member 32 that produces greater fixation force thanfixation member 30′.

FIG. 3 is a conceptual diagram of pacemaker 10 including pace/senseelectrodes along electrical conductor 16. In some examples, as shown inFIG. 3, one or more ring electrodes 42′ and 46′ may be located alongelectrical conductor 16. Electrode 42′ is positioned proximate toproximal housing end 56 to form a bipolar pair with tip electrode 40.Electrode 46′ is shown proximate to proximal housing end 66 to form abipolar pair with tip electrode 44. Instead of ring electrodes 42 and 46along housings 50 and 60 as shown in FIG. 2, the ring electrodes 42′ and46′ are each electrically coupled to the respective proximate housing 50or 60 to provide a return anode for bipolar pacing and/or bipolarsensing with respective tip electrodes 40 and 44.

In other examples, electrodes 42′ and 46′ may be provided in addition tohousing based ring electrodes 42 and 46 to provide multiple, differentbipolar electrode combinations with respective pacing cathode electrodes40 or 44 for sensing and pacing in a respective heart chamber. In stillother examples, one anode electrode 42′ may be provided along electricalconductor 16 and coupled to housing 50 of capsule 12, which may have ashorter length than capsule 14, and the other anode electrode 46 may bea housing-based electrode as shown in FIG. 2. An electrode 42′ alongelectrical conductor 16 enables greater electrode spacing betweencathode tip electrode 40 and anode electrode 42′ than a housing basedanode 42. Greater electrode spacing may be desired for some sensingapplications.

Electrodes 40, 42, 42′, 44, 46 and 46′ shown in FIGS. 2 and 3 may be,without limitation, titanium, platinum, iridium or alloys thereof andmay include low polarizing coatings, such as titanium nitride, iridiumoxide, ruthenium oxide, platinum black, among others. Electrodes 40, 42,44 and 46 may be positioned at locations along pacemaker housings 50 and60 other than the locations shown.

Each of housings 50 and 60 may be formed from a biocompatible material,such as a stainless steel or titanium alloy. In some examples, thehousings 50 and 60 include an insulating coating. Examples of insulatingcoatings include parylene, urethane, PEEK, or polyimide among others.The entirety of the housings 50 and 60 may be insulated, but onlyelectrodes 40, 42, 44 and 46 uninsulated. In other examples, theentirety of the housing 50 or 60 may function as an electrode, insulatedfrom tip electrodes 40 and 44, instead of providing a localized anodeelectrode 42 or 46.

Control electronics subassemblies 52 and 62 may each house theelectronics for sensing cardiac signals and producing pacing pulses inthe respective heart chambers in which the separate capsules 50 and 60are implanted. As indicated above, housing 60 further includes a batterysubassembly 64, which provides power to control electronics subassembly62 and to control electronics subassembly 52 via electrical conductor16. Battery subassembly 64 may include features of the batteriesdisclosed in commonly-assigned U.S. Pat. No. 8,433,409 (Johnson, et al.)and U.S. Pat. No. 8,541,131 (Lund, et al.), both of which are herebyincorporated by reference herein in their entirety.

FIG. 4 is a conceptual diagram of an alternative arrangement ofpacemaker 10. In this example, the proximal end 117 of electricalconductor 116 is coupled to a lateral side 57 of capsule 12 that extendsbetween proximal housing end 56 and distal housing end 58. Electricalconductor distal end 118 is shown coupled to proximal housing end 66 ofcapsule 14. In some examples, a side connection of electrical conductor116 to one or both of the capsules 12 and 14 may alleviate tension onelectrical conductor 116 after implantation and/or facilitatepositioning of the capsules 12 and 14 at desired implant positions. Theparticular locations at which electrical conductor 16/116 is coupled tothe capsules 12 and 14 of pacemaker 10 may vary between embodimentsaccording to the requirements of a particular implanted configuration.

In the example of FIG. 4, a telemetry antenna 125 is shown extendingalong electrical conductor 116. Antenna 125 is coupled to a telemetrymodule enclosed by housing 50. When a telemetry module is included ineither of capsules 12 or 14, the overall small size of capsules 12 and14 limits antenna length for wireless telemetry communication withexternal device 20. An antenna 125 may extend from one or both housings50 and 60 along electrical conductor 116. The antenna 125 is coupled toa telemetry module enclosed within the respective housing 50 or 60 usingany necessary feedthrough to carry signals cross the housing.

FIG. 5A is a sectional view of electrical conductor 16 according to oneexample. Electrical conductor 16 includes an elongated lead body 15formed of an electrically insulating material, e.g., polyurethane,silicone, polyethylene or other flexible, biocompatible material. Leadbody 15 may include a central open lumen 120 through which at least oneinsulated, electrically conductive wire or cable extends from theproximal end 17 to the distal end 18 of electrical conductor 16 fortransmitting communication signals and/or power between capsules 12 and14.

In the example shown, a data communication line 122 is provided fortransmitted data signals between control electronics subassemblies 52and 62 of capsules 12 and 14. Communication signals transmitted by datacommunication line 122, also referred to herein as “data line” 122, mayinclude pace delivery signals and sensed event signals. For example,when capsule 14 is positioned in the right ventricle, a pace deliverysignal may be transmitted to capsule 12, positioned in the right atrium,each time a right ventricular pacing pulse is delivered by capsule 14.Each time an R-wave is sensed by capsule 14, an R-wave sense signal maybe transmitted from capsule 14 to capsule 12 via data line 122.Likewise, each time an atrial pacing pulse is delivered by RA capsule12, a pacing delivery signal may be transmitted from capsule 12 tocapsule 14. Each time a P-wave is sensed by RA capsule 12, a P-wavesense signal may be transmitted from capsule 12 to capsule 14. In thisway, a control module included in each of the control electronicssubassemblies 52 and 62 is notified when the other capsule is deliveringa pacing pulse or sensing cardiac events such that the control modulescan operate cooperatively in delivering dual chamber cardiac pacing orother pacing therapies.

An insulated power line 126 is also shown extending through lumen 120.Power line 126 may be included in electrical conductor 16 if onecapsule, e.g., capsule 12, is not provided with its own power supply,and both capsules 12 and 14 are powered by a single source, e.g.,battery subassembly 64 of capsule 14. Power line 126 provides a V+ powersignal from battery subassembly 64 of capsule 14 to the controlelectronics subassembly 52 of capsule 12 in the example shown in FIG. 2.A ground line 132 may be included in some examples but may be optionalsince the housings 50 and 60 each provide a local ground connection forcontrol electronics subassemblies 52 and 62, respectively. If the V+power line 126 is present, ground line 132 may be desirable, however, toprovide a circuit return path that is not through the patient's body.

In various examples, electrical conductor 16 may include only one signalline, e.g., data line 122. In other examples, electrical conductor 16includes only V+ power line 126 and ground line 132. The number ofelectrical feedthroughs available for providing electrical couplingacross housings 50 and 60 to signal lines included in electricalconductor 16 may be limited due to the overall size of housings 50 and60. For example, a maximum of three electrical feedthroughs may beavailable to provide connection to a V+ power line 126, one data line122, and one ground line 132.

The one data communication line 122 may be used in a half-duplexcommunication protocol for sending signals from capsule 12 to capsule 14and from capsule 14 to capsule 12 in a non-interfering matter. A singlebit digital communication signal may be transmitted via data line 122for indicating when a pace or sense event occurs in a heart chamber tothe capsule located in the other chamber. In other examples,communication may be “one-way” using a single data line 122. In otherexamples, data line 122 may include a multi-bit data bus fortransmitting data between capsules 12 and 14.

As described in conjunction with FIG. 4 above, electrical conductor 16may include an antenna 125 (not shown in FIG. 5A) extending along aportion of the lumen 120 from a respective housing 50 or 60 in someexamples.

The lead body 15 is shown as a single lumen body in FIG. 5, but may be amulti-lumen body in other examples. A separate data communication line,power supply line and ground line may extend through multiple, isolatedlumens from proximal end 17 to distal end 18 of electrical conductor 16.In some examples, electrical conductor 16 includes at least power line126 for transferring a power signal or at least data line 122 fortransferring a data communication signal so that the power signal or thedata communication signal is carried between the proximal end 17 and thedistal end 18 of the electrical conductor 16, between housings 50 and60, to thereby enable pulse generators enclosed by each of the separatehousings to operate in a coordinated manner for delivering dual chamberor multi-site pacing to heart 8.

FIG. 5B is a sectional view of electrical conductor 16 according to analternative example. In examples that include an anode electrode alongelectrical conductor 16, e.g., as shown in FIG. 3, an electrode leadconductor 130 extends through a portion of lumen 120, from the electrode42′ or 46′ to the respective housing 50 or 60 to provide a returncircuit path. In this example, electrical conductor also includes acommunication data line 122 but does not provide power transmissionbetween the two capsules 12 and 14.

FIG. 6 is a functional block diagram of pacemaker 10 according to oneexample. RA capsule 12 includes a pulse generator 102, sensing module104, control module 106, and memory 110 all enclosed in the housing 50of control electronics subassembly 52. As used herein, the term “module”refers to an application specific integrated circuit (ASIC), anelectronic circuit, a processor (shared, dedicated, or group) and memorythat execute one or more software or firmware programs, a combinationallogic circuit, or other suitable components that provide the describedfunctionality. The functions attributed to a given capsule 12 or 14herein may be embodied as one or more processors, controllers, hardware,firmware, software, or any combination thereof.

Pulse generator 102 generates electrical stimulation pulses that aredelivered to heart tissue via electrodes 40 and 42 under the control ofcontrol module 106. Sensing module 104 receives cardiac signalsdeveloped across electrodes 40 and 42 for sensing cardiac events, e.g.,P-waves in the right atrium. Sensing module 104 passes sensed eventsignals to control module 106. For example, sensing module 104 may beconfigured to sense P-waves in response to the EGM signal received fromelectrodes 40 and 42 crossing an auto-adjusting P-wave sensingthreshold. Control module 106 controls pulse generator 102 to deliver RApacing pulses as needed to maintain a desired heart rhythm according toa programmed pacing mode and other pacing control parameters.

Pacing control parameters and other operational control parameters maybe stored in memory 110 for access by control module 106. In otherexamples, memory 110 is not included in capsule 12. Memory 210 includedin RV capsule 210 may be used to store RA pacing and sensing controlparameters accessible via electrical conductor 16 by control module 106.

Programmable control parameters used by control module 106 to controlpacing and sensing functions may be received by telemetry module 208 ofRV capsule 14, transmitted to capsule 12 via data communication lines inelectrical conductor 16, and stored in memory 110. RA capsule 12 may notinclude a separate telemetry module in some examples for transmittingdata to/from external device 20. Telemetry communication with externaldevice 20 is performed by telemetry module 208 via electrical conductor16.

RV capsule 14 includes pulse generator 202, sensing module 204, controlmodule 206, memory 210, telemetry module 208 and a power source 214.Pulse generator 202 generates electrical stimulation pulses that aredelivered to heart tissue via electrodes 44 and 46. Pulse generators 102and 202 may include one or more capacitors and a charging circuit tocharge the capacitor(s) to a programmed pacing pulse voltage. Atappropriate times, e.g., as controlled by a pacing escape interval timerincluded in a pace timing and control circuit in control module 106 or206, respectively, the capacitor is coupled to pacing electrodes 40, 42or 44, 46, respectively, to discharge the capacitor voltage and therebydeliver the pacing pulse. Pacing circuitry generally disclosed in theabove-incorporated U.S. Pat. No. 5,507,782 (Kieval, et al.) and incommonly assigned U.S. Pat. No. 8,532,785 (Crutchfield, et al.),incorporated herein by reference in its entirety, may be implemented inpacemaker 100 for charging a pacing capacitor to a predetermined pacingpulse amplitude under the control of control modules 106 or 206 anddelivering a pacing pulse by the respective pacemaker capsule 12 or 14.

Sensing module 204 receives a cardiac EGM signal developed acrosselectrodes 44 and 46 for sensing cardiac events, e.g., R-waves. Sensingmodule 204 passes sensed event signals to control module 206. Forexample, sensing module 204 may be configured to sense R-waves inresponse to the EGM signal received from electrodes 44 and 45 crossingan auto-adjusting R-wave sensing threshold. Sensing modules 104 and 204may each include a bandpass filter, which may be an adjustable filter,having a center frequency and passband selected to filter non-cardiacsignals and improve the signal-to-noise ratio for sensing intrinsiccardiac events.

Sensing modules 104 and 204 may each include a digital convertor thatconverts the EGM signal received across respective electrodes 40, 42 and44, 46 to a multi-bit digital signal. Control modules 106 and 206 mayreceive the multi-bit digital signal from respective sensing module 104or 204 and analyze the digital signal for use in detecting cardiacevents and controlling pulse generators 102 and 202 to deliverappropriate therapy.

Memory 110 and memory 210 may each include computer-readableinstructions that, when executed by respective control modules 106 and206, cause respective RA capsule 12 or RV capsule 14 to perform variousfunctions attributed throughout this disclosure to pacemaker 100. Thecomputer-readable instructions may be encoded within memory 110 ormemory 210. Memory 110 and memory 210 may include any non-transitory,computer-readable storage media including any volatile, non-volatile,magnetic, optical, or electrical media, such as a random access memory(RAM), read-only memory (ROM), non-volatile RAM (NVRAM),electrically-erasable programmable ROM (EEPROM), flash memory, or otherdigital media with the sole exception being a transitory propagatingsignal. Each of memory 110 and memory 210 may store timing intervals,counters, or other data used by control modules 106 and 206 to controlthe delivery of pacing pulses by pulse generators 102 and 202 to providecoordinated dual chamber pacing therapy.

Memory 210 may have a greater storage capacity than memory 110 in someexamples. In order to reduce the overall size of pacemaker 10, and morespecifically reduce the size of one of the capsules, one capsule 12 maybe provided with a reduced storage capacity memory 110. Memory 110 maybe used to store data used by control module 106 used on a frequentbasis to control RA capsule operations. For example, data used bycontrol module 106 for controlling pacing on a beat-by-beat basis may bestored in memory 110. Other data, such as EGM signal episode data, datastored for diagnostic purposes, may be transmitted from RA capsule 12via electrical conductor to RV capsule 14 and stored in memory 210. Suchdata originating from RA capsule 12 may be retrieved from memory 210 fortransmission to external device 20 by telemetry module 208. In this way,larger amounts of data, which may be stored for relatively long time, ordata not needed by control module 106 for controlling RA capsuleoperations on a beat-by-beat basis may be stored in the larger capacitymemory 210 allowing the overall size of RA capsule 12 to be reduced.

Pacemaker 100 may further include one or more physiological sensors 212used for monitoring the patient, such as a pressure sensor,accelerometer, heart sound sensor, etc. In some examples, physiologicalsensors 212 include at least one physiological sensor producing a signalindicative of the metabolic demand of the patient. The signal indicativeof the patient's metabolic demand is used by control module 206 fordetermining a sensor indicated pacing rate to control a pacing rate thatmeets the patient's metabolic demand. For example, sensors 212 mayinclude an accelerometer for producing a patient activity signal, whichmay be passed to control module 206 and/or control module 106. In someexamples, one control module 106 or 206 determines a sensor indicatedpacing rate based on a sensor signal and transmits the sensor indicatedpacing rate to the other control module 106 or 206 via electricalconductor 16 so that both control modules are controlling pacing rate inthe same manner.

Power source 214 provides power to each of the other modules andcomponents of pacemaker 100 as required. Power may be provided frompower source 214 to RA capsule 12 via electrical conductor 16. Powersource 214 may include one or more energy storage devices, such as oneor more rechargeable or non-rechargeable batteries. The connectionsbetween power source 214 and other pacemaker modules and components arenot shown in FIG. 6 for the sake of clarity. In some examples, a V+power line 126 (FIG. 4) extends through electrical conductor 16 and iselectrically coupled to power source 214 and to control module 106 forproviding a V+ signal to control module 106.

Telemetry module 208 includes a transceiver and associated antenna fortransferring and receiving data via a radio frequency (RF) communicationlink as described above. RF communication with external device 20 (FIG.1), may occur in the Medical Implant Communication Service (MICS) band,the Medical Data Service (MEDS) band, or other frequency bands,including, but not limited to a 2.4 GHz industrial, scientific andmedical (ISM) band for Bluetooth and IEEE 802.11 b/g/n standards.Telemetry module 208 transmits data between external device 20 and RAcapsule 12 via a data line 122 (shown in FIG. 5A) included in electricalconductor 16 in the example shown. Data line 122 extending throughelectrical conductor 16 provides data communication between controlmodule 106 of RA capsule 12 and telemetry module 208.

In the example of FIG. 6, each capsule 12 and 14 performs pacing andsensing functions using separate pulse generators 102 and 202, sensingmodules 104 and 204 and control modules 106 and 206. Shared telemetryfunctions with external device 20 via telemetry module 208, shared datastorage in memory 210, and a shared power source 214 are enabled byelectrical conductor 16 that includes a data line 122 and a power line126. By separating some pacing and sensing functions across two separatecapsules 12 and 14, each capsule can be reduced in overall size comparedto a unitary pacemaker housing. By sharing some functions, such aswireless telemetry, data storage and/or power supply, one capsule 12 canbe reduced in size compared to the other capsule 14.

FIG. 7 is a functional block diagram of pacemaker 100 according toanother example. In FIG. 7, electrical conductor 16 that tetherscapsules 12 and 14 together includes only a data line 122 (FIG. 5A) butno power line 126. RA capsule 12 includes a power source 114 forproviding power to control module 106, pulse generator 102, sensingmodule 104 and memory 110. Electrical conductor 16 is used for datacommunication between RV capsule 14 and RA capsule 12 but not for powertransfer.

FIG. 8 is a functional block diagram of yet another exampleconfiguration of pacemaker 10. In this configuration, RA capsule 12includes a telemetry module 108 and memory 110 with sufficient storagecapacity to support RA pacing and monitoring functions. In some examplesRA capsule 12 does not include a power source, e.g., as shown in FIG. 6.Electrical conductor 16 includes a V+ signal line but does not include aseparate data communication line. Electrical conductor 16 provides powerfrom power source 214 of RV capsule 14 to RA capsule 12.

In other examples, RA capsule 12 includes a back-up power source 114 toprovide power to pulse generator 102, sensing module 104, control module106, memory 110, and telemetry module 108, if included. Power source 114may have a much smaller capacity than power source 214 and provides backup power when a short circuit or open circuit condition is detected. Ifpower is lost, back-up power source 114 is configured to provide powerto the other modules and components of capsule 12 as needed. Controlmodule 106 may operate in a power-savings mode, e.g., by reducing ordisabling non-critical device functions.

FIG. 9 is a flow chart of a method performed by pacemaker 100 accordingto one example. At block 202, the implantable pacemaker 100 is advancedvia a delivery tool, e.g., an elongated flexible catheter, to positionthe distal housing end 68 of capsule 14 at a first desired pacing site,e.g. in the right ventricle, and anchor electrode 44 at the desiredpacing site using fixation member 32. Testing, such as pacing capturethreshold, may be performed at block 202 to confirm that theimplantation site is acceptable.

At block 204, the delivery tool is maneuvered to position distal housingend 58 of capsule 12 at a second desired pacing site, e.g. in the rightatrium. It is contemplated that in some examples, capsules 12 and 14 maybe positioned within or along the same heart chamber for pacing atmultiple locations of the same heart chamber, e.g., the left ventricle.The tip electrode 40 is anchored at the second pacing site usingfixation member 30, which may have a different fixation force thanfixation member 32 as described previously herein. The two housings 50and 60 may remain tethered together by elongated electrical conductor 16throughout the implantation process, having a proximal end coupled tothe capsule 12 and a distal end coupled to capsule 14. The two capsules12 and 14 are positioned for delivering pacing pulses to the firstpacing site via tip electrode 44 and ring electrode 46 and the secondpacing site via tip electrode 40 and ring electrode 42. The electricalconductor 16 is used to transfer signals along a signal line extendingthrough the elongated electrical conductor 16 between the first housingand the second housing to enable coordinated delivery of the firstpacing pulses and the second pacing pulses in a dual chamber ofmulti-site pacing mode.

In various examples, the transferred signal enabling coordinated dualchamber or multi-site pacing by the two capsules 12 and 14 may include aV+ signal for power transfer to power a pulse generator enclosed by oneof the housings 50 or 60 from a power source enclosed by the other oneof the housings 50 or 60. If electrical conductor 16 includes a powerline 126 (FIG. 5) for power transfer as shown in FIGS. 6 and 8, thecontrol module 206 of capsule 14 may monitor for a short or open circuitof the power line 126 at block 204. Short circuit monitoring and opencircuit monitoring may include monitoring the electrical current suchthat if a very high current (or low impedance) or very low current (orhigh impedance) is detected a short circuit (or open circuit) isdetected. If a power line short is detected, at decision block 208, thepower transfer is withheld at block 210 by control module 206. Controlmodule 206 may generate an alert signal at block 214 that is transmittedvia telemetry module 208 to external device 20 to notify the patient, aclinician or other user of the short circuit condition.

In some examples, capsule 12 may include a power source 114 that is asmall capacity, back-up power source for use during a short circuitcondition. Control module 106 may enable a power-savings operating modethat limits pacing at a base rate and/or reduces or eliminatesnon-critical device functions. When back-up power source 114 becomes thepower source for capsule 12, a power-on reset condition may occur. Insome examples, capsule 12 may transfer a signal via electrical conductor16 to signal to capsule 14 that back-up power is available. In otherexamples, the pacemaker 100 may be automatically configured to switch toa back-up power operation mode upon detecting a power line short or opencircuit condition that includes withholding power transfer but allowsthe separately powered capsules to continue operating in a coordinateddual chamber or multi-site manner, which may involve transferring otherdata communication signals via a data line of electrical conductor 16 asdescribed below. The process of flow chart 200 may advance to block 220if back-up power is available. If back-up power is not available asdetermined at block 216, control module 206 may switch to a singlechamber or single site pacing mode at block 218.

In other examples, the transferred signal enabling coordinated dualchamber or multi-site pacing by the two capsules 12 and 14 may include apace delivery signal or a sensed event signal. At block 220, if a pacingpulse is delivered by the pulse generator 102 or 202 enclosed by one thehousings 50 or 60, then a pace event signal is transferred at block 222by the respective control module 106 or 206 that controlled the pacingpulse delivery, via a data line included in electrical conductor 16, tothe other control module 106 or 206 enclosed by the other housing 50 or60. Similarly if a cardiac event (e.g., a P-wave or R-wave) is sensed byone of the sensing modules 104 or 204 enclosed by one of the housings 50or 60, the respective control module 106 or 206 enclosed by the samehousing 50 or 60 transfers a sensed event signal via the electricalconductor 16 at block 222 to the other one of the control modules 106 or206 enclosed by the other one of the housings 50 or 60.

At block 224, the pulse generator 102 or 202 enclosed by the other oneof the housings 50 or 60 is controlled by the control module 106 or 206that received the pace delivery signal or the sensed event signal viathe electrical conductor 16. The pulse generator 102 or 202 may becontrolled to inhibit a pacing pulse in response to a received pacedelivery or sensed event signal or trigger a pacing pulse to bedelivered at a programmed pacing escape interval following the receivedpace delivery signal or sensed event signal. The response of the controlmodule 102 or 202 at block 224 to a received signal may depend uponwhich type of signal (pace delivery signal or sensed event signal) isreceived, a programmed pacing mode, and the timing of the receivedsignal relative to a previous paced or sensed event.

In other examples, the transferred signal enabling coordinated dualchamber or multi-site pacing may include retrieving a sensing and/orpacing control parameter stored in a memory 110 or 210. As such, atblock 226, other data may be transmitted between the capsules 12 and 14via the electrical conductor 16 extending between the housings 50 and60. For example, pacemaker 100 may be an interrupt-driven device thatsends or retrieves data at block 226 upon interrupt clock signals. Atblock 226, data may be sent to or retrieved from memory 110 or 210enclosed by one of the housings 50 or 60, respectively, by a controlmodule 106 or 206 enclosed by the other one of the housings 50 or 60.Such data may include physiological data to be stored by the memory 110or 210 of one capsule 12 or 14 or operating control parameters stored bythe memory 110 or 210 of one capsule 12 or 14 and retrieved by thecontrol module 106 or 206 of the other capsule 12 or 14, e.g., when theother capsule 12 or 14 has limited memory storage capacity compared tothe first capsule.

Data obtained or transmitted at block 226 may include data transferredfrom control module 106 to telemetry module 208 via electrical conductor16 or data received by telemetry module 208 from external device 20transferred to control module 106. Such data may include operatingcontrol parameters required by control module 106 that enablecoordinated dual chamber or multi-site pacing pulse delivery by theseparate capsules 12 and 14.

If a short or open circuit condition has not been previously detected atblock 208, the control module 206 may continue monitoring for a powerline short at block 206 during pacing operations performed at blocks 220through 226 as indicated by the return arrow to block 206. It isrecognized that once a power line short or open circuit condition isdetected at block 208, monitoring for a power line short circuit or opencircuit and the responses at blocks 210 through 216 need not berepeated.

Thus, various examples of a pacemaker having two distinct housingstethered together by an electrical conductor have been described. It isrecognized that various modifications may be made to the describedembodiments without departing from the scope of the following claims.

1. An implantable pacemaker, comprising: a first housing enclosing afirst pulse generator configured for delivering first pacing pulses to afirst heart location via at least one first electrode carried by thefirst housing; a second housing enclosing a second pulse generatorconfigured for delivering second pacing pulses to a second heartlocation spaced apart from the first heart location via at least onesecond electrode carried by the second housing; and an elongatedelectrical conductor having a proximal end and a distal end, theproximal end coupled to the first housing and the distal end coupled tothe second housing to tether the first housing and the second housingtogether, the elongated electrical conductor comprising a signal lineconfigured to carry an electrical signal between the first housing andthe second housing.
 2. The pacemaker of claim 1, further comprising: afirst control module enclosed by the first housing; a second controlmodule enclosed by the second housing; the signal line comprising a dataline extending through the elongated electrical conductor, the firstcontrol module configured to send a communication data signal to thesecond control module via the data line.
 3. The pacemaker of claim 1,further comprising: a first control module and a first memory forstoring data enclosed by the first housing; a second control moduleenclosed by the second housing; the signal line comprising a data lineextending through the elongated electrical conductor, the second controlmodule configured to access the first memory via the data line.
 4. Thepacemaker of claim 1, further comprising: a power source enclosed by thefirst housing; the signal line comprising a power signal line fortransferring power from the power source to the second pulse generator.5. The pacemaker of claim 4, further comprising: a control moduleenclosed by the first housing and configured to: test the power signalline for detecting whether one of a short circuit condition and an opencircuit condition exists along the power line, and withhold transferringpower from the power source to the second pulse generator in response todetecting the one of the short circuit condition and the open circuitcondition.
 6. The pacemaker of claim 1, further comprising: a telemetrymodule enclosed within the first housing and configured for wirelesscommunication with an external device; a control module enclosed withinthe second housing; the signal line comprising a data line extendingthrough the electrical conductor, the control module configured to senddata to and receive data from the telemetry module via the data line. 7.The pacemaker of claim 1, further comprising: a telemetry moduleenclosed within one of the first housing and the second housing andconfigured for wireless communication with an external device; whereinthe signal line comprises an antenna coupled to the telemetry module. 8.The pacemaker of claim 1, wherein the first housing comprises aconcavity to create a first fixation force of the first housing aftertissue encapsulation at the first heart location that is greater than asecond fixation force of the second housing after encapsulation of thesecond housing at the second heart location.
 9. The pacemaker of claim1, further comprising: a first fixation member coupled to the firsthousing and having a first fixation force; a second fixation membercoupled to the second housing and having a second fixation force lessthan the first fixation force, wherein the first fixation membercomprises at least one of a first length, a first stiffness, a firstthickness, and a first bending angle that is greater than a respectiveone of a second length, a second stiffness, a second thickness and asecond bending angle of the second fixation member.
 10. The pacemaker ofclaim 9, wherein the first fixation member is a passive fixation memberand the second fixation member is an active fixation member.
 11. Thepacemaker of claim 1, wherein: the first housing comprises a firstsurface chemistry and a first surface geometry, the second housingcomprises a second surface chemistry and a second surface geometry; atleast one of the first surface chemistry and the first surface geometrycontribute to at least one of a faster rate of tissue encapsulation ofthe first housing and a greater final thickness of tissue encapsulationof the first housing than a respective one of a rate of tissueencapsulation of the second housing and a final thickness of tissueencapsulation of the second housing after implantation of the pacemaker.12. The pacemaker of claim 1, further comprising: a first sensing moduleenclosed by the first housing; a second sensing module enclosed by thesecond housing; and a third electrode carried by the electricalconductor and coupled to one of the first sensing module and the secondsensing module.
 13. The pacemaker of claim 1, further comprising: afirst control module enclosed by the first housing and coupled to thefirst pulse generator for controlling the delivering of the first pacingpulses; a second control module enclosed by the second housing andcoupled to the second pulse generator; the signal line comprising a dataline for transmitting a pace delivery signal from the first controlmodule to the second control module, the second control moduleconfigured to control the delivering of the second pacing pulses by thesecond pulse generator in response to the pace delivery signal.
 14. Thepacemaker of claim 1, further comprising: a first sensing moduleenclosed by the first housing and configured to sense a cardiac eventsignal via a pair of electrodes coupled to the pacemaker; a firstcontrol module enclosed by the first housing and coupled to the firstsensing module, the first control module configured to transmit a sensedevent signal via the signal line; a second control module enclosed bythe second housing and coupled to the second pulse generator; the secondcontrol module configured to receive the sensed event signal via thesignal line and control the delivering of the second pacing pulses bythe second pulse generator in response to the sensed event signal.
 15. Amethod performed by an implantable pacemaker comprising a first housingenclosing a first pulse generator, a second housing enclosing a secondpulse generator, the first and second housings tethered together by anelongated electrical conductor having a proximal end coupled to thefirst housing and the distal end coupled to the second housing, themethod comprising: delivering first pacing pulses to a first heartlocation via at least one first electrode carried by the first housing;delivering second pacing pulses to a second heart location spaced apartfrom the first heart location via at least one second electrode carriedby the second housing; and transferring a signal along a signal lineextending through the elongated electrical conductor from the firsthousing to the second housing, the transferred signal enablingcoordinated delivery of the first pacing pulses and the second pacingpulses.
 16. The method of claim 15, wherein transferring the signalcomprises: accessing a memory by a control module via a data line of theelectrical conductor to retrieve a pacing control parameter forcontrolling the delivery of the second pacing pulses, the memoryenclosed by the first housing and the second control module enclosed bythe second housing.
 17. The method of claim 15, wherein transferring thesignal comprises transferring power from a power source to the secondpulse generator, the power source enclosed by the first housing.
 18. Themethod of claim 17 further comprising: monitoring the signal line fordetecting whether a short circuit condition exists along the power line,and withholding transferring power from the power source to the secondpulse generator in response to detecting the short circuit condition.19. The method of claim 15, wherein transferring the signal comprises:sending data to and receiving data from a telemetry module enclosedwithin the first housing and configured for wireless communication withan external device.
 20. The method of claim 15, further comprising:anchoring the first housing at a first pacing site using a firstfixation member coupled to the first housing; anchoring the secondhousing at a second pacing site using a second fixation member coupledto the second housing, the first housing have a first fixation force atthe first pacing site, the second housing have a second fixation forceat the second pacing site that is less than the first fixation force.21. The method of claim 15, wherein transferring the signal comprisestransmitting a pace delivery signal from a first control module enclosedby the first housing in response to delivering one of the first pacingpulses; receiving the pace delivery signal by a second control moduleenclosed by the second housing; and controlling the second pulsegenerator to deliver one of the second pacing pulses in response to thepace delivery signal.
 22. The method of claim 15, further comprising:sensing a cardiac event signal by a first sensing module enclosed by thefirst housing, wherein transferring the signal comprises transmitting asensed event signal in response to the sensed cardiac event signal;receiving the sensed event signal by a control module enclosed by thesecond housing; and controlling the second pulse generator to deliverone of the second pacing pulses in response to the sensed event signal.23. A non-transitory, computer-readable storage medium comprising a setof instructions which, when executed by a control module of animplantable pacemaker, cause the pacemaker to: deliver first pacingpulses to a first heart location by a first pulse generator enclosed ina first housing via at least one first electrode carried by the firsthousing; deliver second pacing pulses to a second heart location by asecond pulse generator enclosed in a second housing via at least onesecond electrode carried by the second housing; and transfer a signalvia an elongated electrical conductor extending from the first housingto the second housing, the transferred signal enabling coordinateddelivery of the first pacing pulses and the second pacing pulses.